Spring means for minimizing undulations in suspension bridges



Dec. 28, 1948. M. k. WOLFARD 2,457,427

SPRING MEANS FOR MINIMIZING UNDULATIONS.

IN SUSPENSION BRIDGES Filed March 1, 1946 3 Sheets-Sheet l Merl R.W0lfa1"d, INVENTOILI BY W l ATTORNEY M. R. WOLFARD Deg. 2s, 1.948.

S FOR MINIMIZING UNDULATIONS IN SUSPENSION BRIDGES 3 Sheets-Sheet 2SPRING MEAN Filed March 1, 1946 M er l W0 lfard INVENTOR.

ATTORNEY M. R. WOLFARD Dec. 28, l948.-

SPRING MEANS FOR MINIMIZING UNDULATIONS IN SUSPENSION BRIDGES 3Sheets-Sheet 3 Filed March 1 1946 QMN 95w QNN MerLRWolfaPd, INVENTOR.

Patented Dec. 28, 1948 SPRING nANs FOR MINIMIZING UNDULA- trrons insuspension BRIDGES Merl R.- Woifard, Cambrid e,- Mass. Application Marchv1, 1946, Serial No. 651,346

Claims. 1 This invention relates to improvements in spring means forminimizing undulations in suspension bridges. v

More particularly it relates to the stabilizing of load-sustainingelements which hang with approximately parabolic sag to sustain deadload at many points between supports, and which are subject tointermittent live loadings that tend to produce undulatory movements ofthose elements, such as wind and trafiic tend to cause in the cables ofsuspension bridges.

The invention provides spring means for yieldably tensioning harnesselements that restrain a principal load sustaining element at apinrality of points spaced apart irom stayed points of that principalelement. The yieldable harness applies forces in directions to opposeundulatory movements of that principal load sustaining element in thatdirection in which the movement of that element there would un-stressthe adjacent stay. These struts.

A principal load-sustaining element that hangs with sag between towersto sustain loading should have a degree of resistance to bending, andthe large cables required for long suspension bridges do have a verysubstantial resistance to localized bending. This resistance may besumcient to resist localized deflections, resulting from live loadings,without being su'fiicient to prevent the longer undulatory movementswhich successive wind impulses and moving loads tend to produce. Theselonger undulations are transmitted to the bridge platform. If notarrested, they may become detrimental, and even destructive. Thecurrently accepted method of restricting such undulations is to add astiiiening girder. Such girders are both massive and expensive. Incomparison, the harness structure of this invention which minimizes suchundulatory movements withoutthe use of stiflening girders, is light andinexp nsive.

The invention is particularly adapted to destroy the eflect of thoserhythmic variations of wind which tend to build up undulations, Whenheavy traffic loads are imposed on such a bridge, deflections of theprincipal load-sustaining elements may be permitted, because therepetitive sequence of these heavy loads is slow, as compared with thefrequency of those undulations which are most troublesome in a bridgestructure. The invention provides means for the prompt and positivearresting of growth of those undulatory movements which the smallerdistorting forces tend to initiate, such growth being troublesome anddangerous in the principal load-sustaining elements of a suspensionbridge.

Therefore it is a leading object of the present invention to restrictswaying and vibrating'movements resulting from the intermittentincidence of live loadings, whether caused by winds or trai stays maybeeither ties or 2 no or both, and especially to restrict the amplitude ofthose movements which would be aggravated by the cumulative influence ofeither or both of these disturbing forces.

The unique effectiveness of the structure of the invention depends uponthe spring loading of harness elements; and the locations of thehartress-connected points, along a load-sustaining principal element, inrelation to stayed locations along that element, combined with thedirection in which "the harness acts to restrain initiated movements ofthat load-sustaining element. An important result of this elasticrestraint at harness connected points is that it effectively arrestslengthwise travel of, undulations, along the principal load-sustainingelements of the suspension bridge. Rhythmic deflections resulting from10- caiized impulses do not become communicated to remote partsoftheprincipal elements. Therefore successive gusts of wind cannot buildup large undulations.

Each elastically-restrained harness-connected point along theload-sustaining element is preferably at the vicinity of a stayed pointthereof, and yet is at a distance from that stayed point such that theintervening length of element will permit appreciable flexure of theelement so that when this element is static it "is deflected,elastic'ally by the harness, in direction opposite to that in which itis held deflected by the adjacent stay.

In my copending application for patent Serial No. 576,394, now PatentNo. 2,433,878, I point out the value of deflecting a principalload-sustaining element upward and downward at certain spaced apartpoints along the length of that element. In

this present application I disclose animprovemerit comprising springmeans, for elastically deflecting at least. one such point in eachendhalfeportion of the span; When the spring means is connected toprovide only one such elastically deflected point, thatpoint ofconnection preferablyshould be toward the center of the span from thatone of the inclined stays which is nearest the center of the span.

The spring may be located at or close to .a stayed point of theload-sustaining element, but preferably it is at such a distance from astayed point as will permit appreciable 'flexure of that element, andyet be close enough to the stayed point to prevent any large undulationsof the element at that location. In any case each spring elasticallyapplies a force at the vicinity of a stayed point, or at the vicini'tiesof several stayed points. These points may be stayedby ties only, or bycertain arrangements of both ties and struts. An arrangement of both isdisclosed, which is particularly advantageous for stabilizing side spansin a, suspension bridge, as well as other combinations of ties andstruts which have especial characteristics.

An important feature of the invention is that the springs are supportedand connected so that the harness elements, which transmit the elasticforces to the harness connected points of the principal load-sustainingelement, are protected against excessive stressing. When such a har nesselement extends through the middle portion of the span, above theload-sustaining element each principal load-sustaining ;.element it,which may be a cablejexten'ds' overthe towers l2 to anchorages M. Thebridge platform it is suported by hangers l8. In each end-third-portionoifthespanfbetweentowers there is a tie H exand is connected to thatprincipal element at locations which are near the connections oi theinclined stays in the end portions of the span,

then changes in temperature cause those locations to move slightly awayfrom, or toward, the tower, 1

as the temperature rises or falls respectively. The invention providesspring loaded lever means for maintainingthe tension in such a harnesselement approximately constant, irrespective of changes in temperatures;I

It is intended to frame'the'appendedclaims so that they willparticularly point out all features of patentable novelty which exist inthe disclosure herein made ofjthe employing of elastic means fOrthepurposes stated. The accompanying drawings, which are diagrammatic,illustrate several embodimentsof the invention; but it will beunderstood that engineers can make various modifications withinthe'scope of, the claims. a i 1 Figure 1 is a sideelevation of asuspension bridge in which the uppe'rharness element extends ,fromjoneanchorage totheother;

Figure 2 is an end elevation in cross section, through the principal.load-sustaining element and the harness element showing the spring meansby which they'are connected, in Figure' 1 and Figure. 3; v i 3 Figuresis a side elevation of-a suspension bridge in which the upper harnesselement extends from anchorage to anchorage, but is dif ierentlyconnected to the principal load-sustaining element; f .1 a

Figure 4 is a side elevation of a suspension bridge. in which the upperharness element ex; tends less than the fulllength of the main span;

Figures 5 and 6 are cross sections oi structural shapes that may beemployed for the principal load-sustaining element in Figure 4; I 7

Figures 7,78 and 9 arerespectively a plan, a side elevation, and an endelevation ins section on the line 99 of Figure 8, rotated 90, showingthe spring means which is in Figured, greatly enlarged; v f I Figure 3ais an elevation, comparablei to'Figure 8, of a fragment of analternative structure, using a push rod;

Figure 10 is a side elevation of "a portion, and Figure "11 is a likeelevation on' a smaller scale of the full length, of nearly similarsuspension bridges,in which one of thestays at each'end of J the spanisa bent inclined strut;

Figure 12 is a side elevation of a span between towers of a suspensionbridge in which the upper harness element extends less than thelength ofthe span; I I Figure 13 is a side elevation of a fragment of a bridgesimilar to Figure 12, diiierently arranged in its middle portion; 7 jFigures 14 and 15 are respectively end and side elevations of a springmeans of Figure 10 enlarged; d p

Figure 16 is an end elevation of an arrangement of spring meansindicated in Figures 11, 12 and s r 1 r Figure 17 is an end elevation.of spring means shown in Figure 13; and

. are two inclined nest] and 53, the latter being tending from theprincipal element downward, at

aninclinatoathe tower; and in each side span there is a similar inclinedtie l5. An upper harness element-20 extends above the principalloadsustaining element It except that near the anchorages it crosses tobelow that element, and it is anchoredbelow that element. There is alower harness element ZZie tendi'ng through the midportionof thespanfclamped to the upper/harness element 2B, by two clamps 23, each atadistance ,from t e center of the span, each end por tion of this"harness element 22 being inclined downward irorri thecla'rn'p 23 andfastenedto the principal element l0; This lower harnesselemerit'isfltensioned to depress the upper harness e ement'gjatiachfclamp, 1'23, and 'to deflect the principal element illupwardateachend oi the harness element 22. Ateachlclamp 23 a strut 2 5is represented, which may besetthere for deflecting the principalelement" It downward at the lower.v end ,of this strut. [This strut maybe omitte d,ii .,desired, V p ...',j ,Spri ngfineans,gil s'located atthevicinity of the inclined. tie i iborirlects the principal element 7 itwith: the upper harness elefmentZtl and provides a deflection, in} that.upper. harness i element there. Figure Zjshows apracticablejform inwhich this spring means 30 may be embodied for drawing elastically, the,harness element 20 toward the principal element, ,1 0, being a UV-boltenclosing both elements, with compression springs tending todiiriinishtheilheight or .the loop.

;..L Infl'e'ach sidefspan' thereis short 'at'ithevicinity, of theinclined tie i 5', connecting theprirl'cipalelementljil withthe harnesselement 20 ,"andldeflecting the harness element downward there...

f In FiguresQtheprincipal load sustaining paemen-t isfmarkediilll, theupper harness element Til,

the. iewer harffess" element'IZ and the spring meansIIiOQ ,The inclihedtie '55 inthe side spans corresponds to thatmarkediiain Figure ,1. Ineach endthirdiporti-on of the main span there the further from thetower-52. The spring means 3971s located between these ties and there isanother tfie""8 l between-the upper harness element 10 and .theprincipalload-sustainingelement 50, connected nearer "the center of thel-sp'anthan is the tie 53,and tensioned toproduce a slight de fiection downwardin the upper harness element 10. YIn' this figure the lower harnesselement fl2.is shown clamped to the upper harness element 10 at 13atthel'c'ejnter of the 'spa'rL:

' In Figure 45 the principal load-sustaining element H0 is a structuralchannel as indicated in cross section in Figure 6. At each tower thereare. three inclined ties i ll, H3 and H5 arranged similarly'tof'those inFigure 3'... The upper harness element [20 extends from locations l2! inthe principal load-sustaining,jelement which are between the two ties.n,l;' 'l l1s in each end-third;

i ise n i d t il. jFisi res 8' and. i oc d the inclined tie u 3. By theat the" Vicinity" 0f tie Si l coated e wholc esisn ediil fl n species ofspring means thus shown the tension in the upper harness element 128 maybe main tained substantially constant, notwithstanding that temperatureor other cause changes the length of the upper tension element I 20 andthus affects its degree of deflection. Any suitable fulcrum mock M3mounted on the principal loadsust'aini'ng element H0 has a lever whosepower arm I42 is loaded by a tension spring I which is anchored at alittle distance along the load-sus* taining element at I46. The work armof this lever has a link 148 extending up to the harness element I20from the pivotal point 141 of the work arm. As portrayed, the lever isarranged and located with its fulcrum I 4|, power arm I42, work arm I41,and the spring anchorage I46, so that the harness element I20 is seendeflected to its lowest positionats when its temperature is highest andits length great-est. The effective work arm of the lever is relativelyshort. When the lever moves in the direction of the arrow in Figure 8,elongating the spring I44, the effective length of the spring power armdecreases and the effective length of the work arm increases, thusdecreasing the leverage ratio. That is, the tension in the link I 48decreases as the deflection of the harness element H0 decreases, therebytending to maintain a uniformity of tension in the harness element I20as that element with reduction of temperature moves toward a position ofless deflection. The deflecting load applied by the spring means I40becomes diminished as the degree of deflection decreases. By this meansa uniformity of tension in the harness element I20 can be approachednotwithstanding changes in temperature which would cause slight movementof each end connection I2I toward and away from its nearer tower. Thismovement would be accompanied by a rise or fall respectively of themiddle portion of the load-sustaining element H0. When the bridgestructure is arranged so that it is desirable to push upward fortensioning the harness element, as in Figures 10, 11, 12, then the linkI48 may be a push rod 8a, as seen in Fig. 8a.

The lower tension element I22 in Figure 4, clamped to the upper tensionelement I20 at the middle of the span, extends thence in each directiondownward to points I25 on the principal load-sustaining element. Thesepoints 125 are each located at a distance toward the center of the spanfrom the inclined tie I is, this distance being less than one-half ofthe distance from that tie to the center of the span, and yet preferablybeing great enough so that the spring mean can cause an appreciableupward deflection in the principal load-sustaining element H0. Tensionin this lower harness element deflects the midportion of the upperharness element I20 downward. It will be understood that in any casewhere a harness element requires a particular tension relative to someother structural part, conventional means for adjusting the length maybe used, such as turn buckles (not shown).

In the side spans represented in Figure 4 there is a stay I ll, which inthis case is a strut, which rests in top bearings and bottombearing thatare rounded to permit slight movement of its upper end lengthwise of thebridge. This deflects upward the principal element H0. Preferably thelength of this strut should be such that this deflection stops short ofreaching the position of a straight index line (not shown) which mightbe drawn between the locations of the principal element at it'sanchorage and at its connection with the the H5 which is between thisstrut and the tower. This conduces to stability and also avoids theimposing of excessive load on the strut. There may be spring tensionmeans 4 l9 connected to the principal element near the upper end of thisstrut and extending downward to -an anchor age.

For the principal load-sustaining elements, in a suspension bridge,cables are preferable if the span is long, because in a long span thesize re quired for carrying the total tension load only is sufficient toprovide a satisfacto-ry localized stiifness of the load-sustainingelement, between the spaceti a'part harness-restrained points. Inshorter spans the size of cables required for carrying the tension loadmay be not sulficient to produce the desired stiffness between thosepoints. in such shorter spans it is desirable to provide a distributionof material which produces the required resistance to bending with aminimum of weight of material. If the load-sustain ing element is achannel bar, used with the channel upward as indicated in Figure 6, therequired stillness between the harness restrained points is obtainedfrom the flanges of the channelwith the use of but little material. Thisresults loccause the major loading is carried by tension; and the webpart of the channel, which carries the tension, is much larger than theupper edges of the flanges which carry the compression. ,Another meansof providing this larger area for carrying tension is to use a T bar,with the stem of the T upward, as indicated in Figure 5.

In Figure 11 each side span has two ties 2I5, ZIB, and two struts 2H,MB. This construction may be used when a relatively long side span isadvantageous, the ties making deflections downward and the struts makingdeflections upward. Preferably the upward deflections at the struts donot rise above the limits indicated withreference to the struts inFigure 4. Between the towers, in each end-third portion of the span,there is a tie 2H providing downward deflection of the principalload-sustainingelement 2+0; and, at a substantial distance from the tie2| I there is a strut 209 producing upward deflection of that element.This strut preferably is set with a bend at 2-08, in its middle portion,as clearly seen in Figures 11 and 10. From this set bend a tie 2'0?extends upward to the principal element 210. A harness element 220extending across the center 01 the span has its ends fast tovthesestruts at their bends 208. Spring means diagrammatically indicated at230, in Figure 11, which may be of the general type 23'2seen in Figureslei and 15, or Y of the lever type seen in Figure 8a, supported on theprincipal element 210 at a distance toward the center from the strut209, deflects the harness element 220 upward, and to a lesser degree de-"fiects the principal load-sustaining element 2H1 downward.

In Figure 10 the harness element 225 is secured to the principal element210 at 226,, which is a little way toward the center from the top of theinclined strut 2'09; and at each end this harness element has anextension 22! from the principal element at 226 to the bend "208 in theinclined strut 209. Spring means 232 supported on the principal elementnear the secured ends oi the harness element 225 deflect thatharn'esselement upward. Each extension 22! of this harness element has aspring 234 pulling it into an upward deflection toward the upper end ofthe strutfils.

The embodiment shown in Figure "12 has in each end portion of theprincipal e1ement'250 a enema? plurality of ties inclined downward tothetower,

that inclined tie 253 which is nearer the center being securedto a cableband 260. The harness element 210 extends along and above the middleportion of the principal element, and its end portions reach to thesecable bands 260, each being looped around a saddle groove in a cableband 260 shown in plan in Figure 18. There is another cableband 262,located far enough from the band 266 to permit of there being a flexurein the principal element between the two bands; and this band 262has-lugs 264 at each side, under which the harness element passes,thencerising and passing over spring means 280 which are preferablylocated at less than half way from the low point 26!; to the center ofthe span. These lugs 264 deflect the harness element downward, and thespring means 280 deflects it upward. The end section of this harnesselement, between the cable bands 262 and 269, may be omitted providedthe harness element is secured to the cable band 262, and provided thatthat band 262, or other connecting means at that location, holds theharness element 210 against lengthwise slip along the principal element259.

The embodiment in Figure 13 is like that in Figures 12 and 18 exceptthat there is a single upward deflection oi the harness element 320 byspring means 339 located at the center of the span.

Figures 14 and 15 illustrate a type of spring means 232 which may beused for tensioning the harness element 225 as shown in Figure 10.

Figure 16 is anend view of spring means such as may be employed in thestructure of Figures substituted for the type 34] shownin Figures 1 and3, or at any other similar location where it is desirable to obtain anoptimum of control with a minimum or" weight, in the harness elements.The springs shown in Figures 10-13 all apply an increase in tension tothe harness elements with a drop in temperature, but their flexibility.may be made to be such that the increase in stress will not beexcessive. The push rod type of Figure 8a may be used to reduce the pushas the deflection decreases. Y i

The bridge structures shown in Figures 10, 11 and 13 are not especiallyadapted for long bridges; they are more particularly applicable toshorter bridges where a light and stable structure is desired.

In all cases the spring tensioned harness pro vides a cushionedrestraint at its points of connection to the load-sustaining element.That is,

the arresting of movement is not abrupt, or jerky, as it may be when anundulating wave in'the load-sustaining element meets the end of a strut,or when an undulating wave relaxes a tie, and then jerks it taut again.It is this jerky phenomenon that the invention is primarily intended toprevent, especially at that one of the inclined ties which, in each endportion of the span, is

nearest to the center of the span. Thus, I believe, this inventionprovides a stabilization er, the carrying structure of a bridge whichminimizes undulations to the extent that sudden excessive stressing inany part of that structure is impossible; and, therefora ascompared withbridges as heretofore built, thebridge structure can be made lighter;the liieoi the bridge will be longer;

and the comfort of persons riding over it will be greater.

- I claim:as my invention:

1 In abridging structure, means for minimizing ;undulatorymovements of aload-sustaining principal element said element being anchored at itsends and'supported upon intermediate towers, with a span between thosetowers having in each end-third-portion of the length of the spanatleast one stay which is fast to theprincipal element, inclineddownward, and fast to' the tower -said means comprisingin' combination,a tension element extending above and along said principal element, atleast through the I heldr'egion of the span, said tension-element beingheld at its ends against lengthwise slip relative to the principalelement; and'spring means en gaged between said principal and tensionelements for deflecting yieldably, the tension element, the yieldableforce being applied against the principal element in a direction tooppose those undulatory movements of that principal element which are ina direction that would tend to unstress saidstay. a V

7 2. Ina bridging structure, means for minimizing undulatory movementsas in claim 1, in'which the spring means engages the principal elementat a locationwhicli is spaced apart from a said stayfwhereby when thestructure is static, fiexure of said principal element is produced. 7 i

3. In a bridging structure,means for minimizing undulatory movements asin claim 1, further characterized in that the spring means engages theprincipal element at a location which is adjacent to a said stay and isfurther from'the tower than is that said stay. a

Q 4;"In a bridging structure, means for minimizing undulatory movementsas in claim 1,'in which each end-third-portion of the span there is aplurality of said stays, spaced apart from each other along'theprincipal element, and in which in each end-'half-portion of the spanthe spring means engage'sthe principal-element'at a location which isfurther from the nearer tower than is that one of said stays which isthe more remote from that tower. v

5. In a bridging structure, means for minimizing undulatory movements asin claim 1, further characmrized inthat the said springmeans comprises alever whose power arm is' loaded by'a spring, whose fulcrum is fastenedto the said principal element, and whose work arm is'connected to theengaging means which deflects the tension element; the leverage beingarranged for its ratio to decrease with that movement of the lever whichattends a decrease in the deflection of the tension element.

' MERL R. WOLFARD.

, REFERENCES CITED The following, references are of rec'ordfin the file,of this patent:

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